The present invention relates generally to the coating of various objects. More specifically, this invention relates to the coating of vapor-deposited poly(p-xylylene) and derivatives thereof onto substrate surfaces and the articles so produced.
High quality uniform coatings on substrates have not been readily achieved with the coating processes known in the prior art. For example, production batches of coated substrates formed by dipping or spraying with a solution or hot melt contain an undesirably high quantity of agglomerates, particularly when thick coatings are applied, and individual articles which are not uniformly or completely coated. Further, it is difficult, if not impossible, to remove from a batch any articles having non-uniform or incomplete coatings. Articles may also be coated by tumbling them with coating materials in rotating barrels. The articles to be coated and the results obtained with this technique are limited. Control of the coating thickness is extremely difficult, especially when the article to be coated has an irregular shape. In addition, excessive agglomeration, considerable stocking of the wetted articles to the barrel walls and substantial barrel wall cleaning difficulties occur.
While ordinarily capable of producing good quality coatings, conventional coating materials and processes suffer serious deficiences in certain areas. The coatings of most coated objects deteriorates rapidly when exposed to atmospheric conditions, abrasion, chemicals, solvents, and the like. Deterioration occurs when portions of or the entire coating separates from the coated object. The separation may be in the form of chips, flakes or entire layers and is primarily caused by fragile, poorly adhering coating material which fails upon atmospheric exposure, impact, and abrasive contact with other particles. Objects having coatings which tend to chip and otherwise separate from the substrate must be frequently replaced thereby increasing expense and loss of durable life. Thus, generally, coated articles having coatings which tend to chip or separate from the substrate cannot be reused or have poor life qualities. Poor life quality occurs when substrates having damaged coatings are not replaced. Many coating materials having long durability either do not adhere well to the substrate or do not possess the desired viewing characteristics. In addition, articles having discontinuous coatings generally promote adhesion failure between the substrate and the coating materials giving rise to the aforementioned problems and result in variations in lifetime characteristics, premature aging of the coated surface causing degradation of the article, scratching of the surface, not to mention difficulties in repairing articles having discontinuous coatings.
Thus, one of the most severe problems encountered in the commercial application of coatings to substrates has been the difficulty of acquiring strong adhesion of the coating material to the surface of the object to be coated. Most coating materials do not exhibit satisfactory adhesion, especially to a wide variety of substrate surfaces such as metals, plastics, and glass. In the past, specific treatments such as physical or chemical etching of the surface have promoted improved adhesion of the coating material. However, such treatments have been laborious, time-consuming, expensive and of limited benefit.
As alluded to above, the provision of thin, uniform films on various objects has not been met with satisfactorily by prior art materials and processes. The production of thin, uniform coatings from polymers has been attempted by a number of techniques such as; deposition and polymerization of a monomer by means of a glow discharge, deposition from solution, deposition from bulk polymerization by thermal evaporation, and deposition from bulk polymerization by radio frequency sputtering. Films of polymers have been prepared by a number of other methods and are generally suitable for a limited number or range of polymeric materials. However, the formation of polymer films which are transparent, colorless, and tough would be extremely desirable. Among the many polymeric coating materials available, the epoxies, urethanes, and silicones have achieved wide acceptance. These coatings are used extensively for protection of substrates from adverse environmental conditions during storage and operating life. Their most important function is to provide the substrate with a moisture and gas barrier as to prevent degradation of the substrate such as due to corrosion.
However, most such organic polymers are permeable to moisture to some extent and cannot be considered true hermetic seals. It is sometimes the case that leakage of water vapor occurs predominantly through pinholes and other imperfections in the barrier film. These may arise during the drying or curing cycle as a direct result of the tendency of evaporating solvents to leave voids or of solidifying fluid systems to draw away from sharp projections of the coated object. To avoid these problems, multiple coatings are often required which add to processing time and labor costs, or thick layers are applied which add weight.
Therefore, a coating process and material that would be highly controllable as to deposition thickness and uniformity is highly desirable. Further, such coating process and material should eliminate pin holes in the coating, and the coating should also conform to the substrate to provide a continuous, uniform barrier. In addition, the coating should possess superior thermal properties allowing exposure to elevated temperatures for extended periods of time. Further, the coating should be highly resistant to chemical and solvent attack. Finally, such coating should have excellent adhesion and have the ability to be deposited as continuous coatings at thicknesses of from less than about 500 Angstrom units to about 5 mils. Thus, the types of coating materials and methods employed for making coated articles having the aforementioned properties and characteristics are limited.
Thus, it will be appreciated that there is a continuing need for improved coatings for numerous articles.